Secondary batteries now used in mobile phones and notebook computers are mostly lithium ion secondary batteries, due to a higher energy density than other secondary batteries. With the latest tendency of mobile phones and personal computers toward multifunctionality, power consumption of these devices has shown a remarkable increase. Therefore, the demands for higher capacity secondary batteries have been increasing. As long as the present electrode active materials are used, it would be difficult to meet the increasing demands in the near future.
Lithium ion secondary batteries generally use graphite as a negative electrode active material. Now Sn alloys and Si alloys which offer 5 to 10 times the capacity potential of graphite are being actively developed. For instance, it has been proposed to produce Sn—Cu-based alloy flakes by mechanical alloying, roll casting or gas atomization (see J. Electrochem. Soc., 148 (5), A471-A481 (2001)). Production of Ni—Si-based alloys and Co—Si-based alloys by gas atomization etc. is also proposed (see JP-A-2001-297757). While these alloys have high capacity, they have not yet been put to practical use on account of the problems of large irreversible capacity and short cycle life.
There is an attempt to use copper foil, which is used as a current collector, electroplated with tin, as a negative electrode (see JP-A-2001-68094). On the other hand, though silicon has higher capacity potential than tin, there is no report on the development of silicon-containing plated copper foil for use in lithium ion secondary batteries because silicon is an element incapable of electroplating.
The aforesaid Si alloys and Sn alloys and, in addition, Al alloys are negative electrode active materials exhibiting high charge and discharge capacities. However, they have the drawback that they incur large changes in volume with alternate repetition of charging and discharging and, as a result, undergo cracking and pulverizing and finally fall off the current collector. To address this problem, techniques for preparing a negative electrode, of which the active material is prevented from falling off, have been proposed, in which a mixture of a negative electrode active material containing Si or an Si alloy and an electro-conductive metal powder is applied to a conductive metal foil, followed by sintering in a non-oxidative atmosphere (see JP-A-11-339777, JP-A-2000-12089, and JP-A-2002-260637). It has also been proposed to prevent fall-off of a negative electrode active material by forming a thin film of Si on a current collector with good adhesion by plasma-enhanced CVD or sputtering (see JP-A-2000-18499). Moreover, extensive studies have been devoted to development of various Sn or Si-based intermetallic compounds (see JP-A-10-312804, JP-A-2001-243946, and JP-2001-307723). Even with these techniques, however, it is still impossible to perfectly prevent fall-off of the negative electrode active material from the current collector as a result of cracking and pulverizing of the active material, accompanying charge and discharge of a secondary battery.
JP-A-8-50922 proposes a negative electrode having a layer containing a metallic element capable of forming an alloy with lithium and a layer of a metallic element incapable of forming an alloy with lithium. According to the disclosure, this layer structure prevents the layer containing the lithium alloy-forming metal element from cracking and pulverizing accompanying charge and discharge of a battery. Judging from Examples of the publication, however, since the thickness of the layer of the metallic element incapable of forming a lithium alloy, which is the outermost layer, is as extremely small as 50 nm, there is a possibility that the outermost layer fails to sufficiently coat the underlying layer containing the lithium alloy-forming metal element. If so, the layer containing the metallic element capable of forming a lithium alloy cannot be sufficiently prevented from falling off when pulverized with repeated charging and discharging of the battery. Conversely, if the layer of the metal element incapable of forming a lithium alloy completely covers the layer containing the lithium alloy-forming metal element, the former layer would inhibit an electrolyte from passing through the latter layer, which will interfere with sufficient electrode reaction. No proposal has ever been made to satisfy these conflicting functions.
Besides the aforementioned, current collectors with appropriate surface roughness and current collectors having micropores that pierce the thickness are known to be used in lithium ion secondary batteries. For example, JP-A-8-236120 proposes a current collector formed of a porous electrolytic metal foil having pores winding across the thickness and making a three-dimensional network. The porous electrolytic metal foil is produced by a process including the steps of electrodepositing a metal on the surface of a cathode drum to form an electrolytic foil of the metal and separating the foil from the drum, wherein an oxide film having a thickness of at least 14 nm is formed on the surface of the cathode drum exposed after separation of the foil, and electrolytic metal foil is deposited on the oxide film. The porosity and pore size of the metal foil are dependent on the thickness of the oxide film formed on the cathode drum. However, since the oxide film comes off little by little, together with the foil, it is difficult to control the porosity and pore size. Additionally, since the pores have a relatively small diameter and form a three-dimensional network, active material paste applied to one side of the foil and that applied to the other side hardly come into contact with each other. There seems to be a limit, therefore, in improving the adhesion between the paste and the foil.
In order to solve the problems associated with the above-described metal foil, Applicant previously proposed a porous copper foil formed by electrodeposition such that copper grains, having an average planar grain size of 1 to 50 μm, are two-dimensionally bonded to one another. The porous copper foil has an optical transmittance of 0.01% or higher and a surface roughness difference of 5 to 20 μm, in terms of Rz, between the side in contact with a cathode for foil formation and the opposite side (see WO 00/15875). When the copper foil is used as a current collector of a lithium ion secondary battery, the following advantages are offered. (1) Since an electrolyte is able to pass through the copper foil so easily, even a limited amount of an electrolyte is permitted to uniformly penetrate into an active material. (2) The copper foil hardly interferes with donation and acceptance of Li ions and electrons during charge and discharge. (3) Having proper surface roughness, the copper foil exhibits excellent adhesion to an active material. According to the process of making the porous copper foil, however, the electrolytic copper foil deposited on a cathode drum and separated from the drum, is subjected to various processing treatments, which make the copper foil unstable. Therefore, the process cannot be seen as satisfactory in ease of handling the foil and fit for large volume production. Additionally, a nonaqueous secondary battery using a negative electrode, prepared by applying a negative electrode active material mixture to the porous copper foil (a current collector), still has the problem that the negative electrode active material tends to fall off accompanying intercalation and deintercalation of lithium, resulting in reduction of cycle characteristics.